Atomic Scaled Depth Correlation to the Oxygen Reduction Reaction Performance of Single Atom Ni Alloy to the NiO2 Supported Pd Nanocrystal

Abstract This study demonstrates the intercalation of single‐atom Ni (NiSA) substantially reduces the reaction activity of Ni oxide supported Pd nanoparticle (NiO2/Pd) in the oxygen reduction reaction (ORR). The results indicate the transition states kinetically consolidate the adsorption energy for the chemisorbed O and OH— species on the ORR activity. Notably, the NiO2/Ni1/Pd performs the optimum ORR behavior with the lowest barrier of 0.49 eV and moderate second‐step barrier of 0.30 eV consequently confirming its utmost ORR performance. Through the stepwise cross‐level demonstrations, a structure–E ads–ΔE correspondence for the proposed NiO2/Ni n /Pd systems is established. Most importantly, such a correspondence reveals that the electronic structure of heterogeneous catalysts can be significantly differed by the segregation of atomic clusters in different dimensions and locations. Besides, the doping‐depth effect exploration of the NiSA in the NiO2/Pd structure intrinsically elucidates that the Ni atom doping in the subsurface induces the most fruitful NiSA/PdML synergy combining the electronic and strain effects to optimize the ORR, whereas this desired synergy diminishes at high Pd coverages. Overall, the results not only rationalize the variation in the redox properties but most importantly provides a precision evaluation of the process window for optimizing the configuration and composition of bimetallic catalysts in practical experiments.


Supplementary
Supplementary Note S1 | Detailed descriptions and explanations concerning the structural designs and the patterns and positions of the doped Ni atom(s) for the nine proposed model catalysts.
As illustrated in Fig. 1 (a), the benchmarking Pd(111) surface model composed of six layers of Pd atoms is labeled as the reference group. In Fig. 1 (b) to (c), the Ni atom(s) are intermixed in the 3 rd layer of the Pd atoms at the NiO 2 /Pd interface for the NiO 2 /Ni 1 /Pd and NiO 2 /Ni 2 /Pd surface models, respectively. Fig. 1 (d) and (e) show that three and four Ni atoms are intermixed in the 3 rd layer of the Pd atoms shaping into an upright-like trigon and a pyramid-like tetragon for the NiO 2 /Ni 3 /Pd and NiO 2 /Ni 4 /Pd models, respectively. Regarding the NiO 2 /Ni 7 /Pd model, six Ni atoms are doped in the 3 rd Pd-layer to shape into a large trigon and one Ni atom is in the 2 nd Pd-layer above the center of Ni trigon, as shown in Fig. 1 (f). While, for the NiO 2 /Ni 10 /Pd model, seven Ni atoms are doped in the 3 rd Pd-layer to shape into a large hexagon and the three Ni atoms are in the 2 nd Pd-layer as a trigon above the center of Ni hexagon, see Fig. 1 (g). As regards the NiO 2 /Ni 13 /Pd model in Fig. 1 (h), seven Ni atoms are in the 3 rd Pd-layer forming a hexagon, three Ni are doped in the 2 nd Pd-layer as a trigon above the center of the Ni hexagon, the remaining three Ni atoms are incorporated into the 1 st Pd-layer to constitute another trigon right above the Ni trigon in the 2 nd Pd-layer, i.e., the NiO 2 /Ni 13 /Pd catalyst is the only model with three Ni atoms occupying (exposed on) the outermost surface. In the cases of the NiO 2 /Ni 1ML /Pd and NiO 2 /Ni 2ML /Pd models in Fig. 1 (i) and (j), the lowest one and two Pd layers are totally substituted by Ni atoms, which are regarded as the one Ni monolayer occupied (Ni 1ML ) and two Ni monolayer occupied (Ni 2ML ) models, respectively.

Supplementary Note S2 | Selection of stable adsorption sites of chemisorbed atomic O (O*), OH radical, O 2 and H 2 O molecules for the proposed model catalysts.
Since the ORR is generally a two-substep electrochemical process, which contains a first absorbed oxygen molecule dissociation ( Further, the underlying charge relocation between the interfacial intercalation of Ni SA and its surrounding atoms in the Ni 1 system was evaluated by charge density difference and Bader charge calculations on behalf of the NiO 2 /Ni n /Pd series. From Fig. 4 (f) and (h), the 3D plots of charge density difference exhibit a significant tendency of charge accumulation congregating between the interfacially intercalated Ni SA and its surrounding Pd atoms via different perspectives. In particular, the charge redistribution is found to be agglomerated over the doped Ni atom, which directly affects the adsorption properties of the surface triatomic Ni-fcc site right above the Ni SA and significantly improves the redox ability of the triatomic site and its neighboring regions, since the nature of the catalysis is the charge exchange between the catalyst and its adsorbates. More explicitly, extra available charge injected to the Ni-fcc site is believed can enhance the capture capacity of the electroneutral O 2 and H 2 O to promote their chemisorption, simultaneously, introduce/tune the repulsive force for the electronegative O* and OH to facilitate their desorption from the catalyst surface, therefore, synergistically boost the two ORR-steps moving forward in the positive direction.
In addition, the Bader charge population was also calculated to quantify the charge transfer of the specific atoms/elements, as shown in Fig. 4 (g), the corresponding specific atoms and locations in the NiO 2 /Ni 1 /Pd model are marked in Fig. 4 (h). It can be observed that one of the three Pd atoms composed of the triatomic Ni-fcc site above the Ni SA (the green bar) possesses an extra charge of 0.03 ethan the Pd atom in benchmarking pure Pd (the orange bar), while, the interfacially doped single Ni atom and the O atom coordinates with it below (the green bars) are also found to have obtained excess charge of 0.85 eand 2.39 ecompared to the Ni atom and O atom in pure NiO 2 (the orange bar), respectively. Consequently, the additional charge obtained to the Ni SA and its above Pd atoms due to the synergistic effect (strain, ligand and geometric effects) enables the surface Ni-fcc site a higher catalytic activity compared to pure Pd benchmark, let alone the core-component NiO 2 octahedron. Since the essence of alkaline ORR catalysis is a process of four-electron transfer between adsorbates and surface metal atoms, meanwhile, oxygen atoms prefer to obtain a charge to satisfy their stable adsorbed state on a metal surface based on the valence charges balance of octet rule in VSEPR model. Thus, more available excess charge within a local domain of the catalyst surface undoubtedly improves the redox behaviors of O*, i.e., the overall performance of the ORR. Additionally, the large increase in the charge of the O atom coordinated with the Ni SA means a much stronger Ni-O electrovalent bond, i.e., a thermodynamically more stable core-shell doping structure. Despite the same surface atomic arrangement of the NiO 2 /Ni n /Pd systems (except the Ni 13 ) as reference Pd (111), the surface charge distribution of the NiO 2 /Ni n /Pd is unbalanced by the interfacially intercalated Ni atom(s)/cluster. Such a phenomenon gives rise to the local domain disparity by changing the chemical identity distribution, thus, the adsorption competition towards the intermediate O* between neighboring hollow sites can be suppressed with effect. These physical phenomena echo and explain the aforementioned changing trends of the and PDOS for the models of interest, as well as the chemical appearance of the significantly improved adsorption properties of the key ORR-species (i.e., O*, OH) in the previous section.

Supplementary Note S4 | Details concerning the stable initial-state (IS), transition-state (TS) and final-state (FS) geometries of the "O 2 dissociation" and "O* hydrogenation" steps on the Fcc (111) facet of the proposed model catalysts.
In terms of the atomic structure investigation and selection for the stable IS and FS of the two stages on the proposed surface models: here, for the 1 st substep "O 2 dissociation", generally, in the first step "O 2 dissociation", the chosen initial-state (IS) and final-state (FS) geometries are respectively an O 2 molecule adsorbing on a diatomic bridge site and then being split into two atomic O* that relocate onto two adjacent hollow sites. In the second step "the O* hydrogenation", the chosen IS and FS are one adsorbed O* atom on the hollow site interacting with a neighboring H 2 O molecule adsorbed on a metal atom. The final product is two OH radicals. The transition-state (TS) structures (the highest NEB image) in the 1 st and 2 nd step reactions are respectively considered as the moments (i.e., the reaction energy  Fig. 6 (a) exhibits the E ads -O* at various adsorption sites of all the proposed model catalysts, where a distinctive fitting crescent-shaped trendline for the E ads -O* distribution is observed from the pure Pd to Ni 13 models. Generally speaking, the E ads -O* enhances from the Pd benchmark to Ni 1 (or Ni 2 ) due to the discrepancy of model configuration and presence of Ni in the Pd-layer, and subsequently increases gradually to the Ni 2ML with the doped Ni atoms increase, which roughly follows the sequence: Ni 1 , Ni 2 , Ni 3 , Ni 4 , Ni 7 , Ni 1ML , Ni 10 , Ni 2ML . Then, the last model Ni 13 is observed to show an evident discrete-distribution with significantly enhanced E ads -O* compared to the rest due to its exclusive configuration of three Ni atoms naked on the outermost Pd surface. Analogical to the E ads -O*, the E ads -OH from Fig. 6(b) basically reflects a comparable tendency of arc-shaped distribution from pure Pd to Ni 13 , with the Ni 2 (or Ni 3 ) as the inflection point. Another aspect, for the impact of the calculated E ads -O*/-OH on the simulated ORR kinetics, both of the barrier distributions of the ΔE1 as well as ΔE2 exhibit the conspicuous crescent-typed fitted trendline for the proposed models from pure Pd to Ni 13 , as shown in Figs. 6(c) and (d). Particularly in Fig. 6(c), the variation trend of the RDS's ΔE1 is found to be almost identical to the change tendency of the E ads -O*, which well proves our predicted dependency between ΔE1 versus E ads -O* mentioned above. As for the ΔE2, it is still observed to feature a roughly semi-arc change tendency in the same sequence from pure Pd to Ni 13 . Though it seems to cause a slightly challenging barrier (ΔE2) on the second stage for Ni 1 (0.3 eV), the impact of its barrier increment is considered marginal compared to the first RDS stage within the calculated thermodynamic coordinates of the integrated ORR path mentioned in Section 3.4.